Solve Using the Quadratic Formula x^2-10x=-21

$x^{2} - 10 x = - 21$

Step 1

Add $21$ to both sides of the equation.

$x^{2} - 10 x + 21 = 0$

Step 2

Use the quadratic formula to find the solutions.

$\frac{- b \pm \sqrt{b^{2} - 4 (a c)}}{2 a}$

Step 3

Substitute the values $a = 1$ , $b = - 10$ , and $c = 21$ into the quadratic formula and solve for $x$ .

$\frac{10 \pm \sqrt{{(- 10)}^{2} - 4 \cdot (1 \cdot 21)}}{2 \cdot 1}$

Step 4

Simplify.

Simplify the numerator.

Raise $- 10$ to the power of $2$ .

$x = \frac{10 \pm \sqrt{100 - 4 \cdot 1 \cdot 21}}{2 \cdot 1}$

Multiply $- 4 \cdot 1 \cdot 21$ .

Multiply $- 4$ by $1$ .

$x = \frac{10 \pm \sqrt{100 - 4 \cdot 21}}{2 \cdot 1}$

Multiply $- 4$ by $21$ .

$x = \frac{10 \pm \sqrt{100 - 84}}{2 \cdot 1}$

Subtract $84$ from $100$ .

$x = \frac{10 \pm \sqrt{16}}{2 \cdot 1}$

Rewrite $16$ as $4^{2}$ .

$x = \frac{10 \pm \sqrt{4^{2}}}{2 \cdot 1}$

Pull terms out from under the radical, assuming positive real numbers.

$x = \frac{10 \pm 4}{2 \cdot 1}$

Multiply $2$ by $1$ .

$x = \frac{10 \pm 4}{2}$

Simplify $\frac{10 \pm 4}{2}$ .

$x = 5 \pm 2$

Step 5

The final answer is the combination of both solutions.

$x = 7, 3$